Method and apparatus for generating an uplink reference signal sequence in a wireless communication system

ABSTRACT

A method of receiving, by a base station, a reference signal in a wireless communication system. The method includes transmitting a cell-specific sequence group hopping parameter to a plurality of user equipments (UEs) in a cell. The cell-specific sequence group hopping parameter is used to enable a sequence group hopping for the plurality of UEs in the cell. The method further includes transmitting a UE-specific sequence group hopping parameter to a certain UE, among the plurality of UEs. The UE-specific sequence group hopping parameter is used to disable the sequence group hopping, enabled by the cell-specific SGH parameter, for the certain UE. The method further includes receiving a reference signal, which is generated based on a sequence group number, from the certain UE. The sequence group number is determined by the UE-specific sequence group hopping parameter.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/521,017 filed on Aug. 1, 2012 (now U.S. Pat. No. 8,804,647 issued onAug. 12, 2014), which is the National Phase of PCT/KR2011/000110 filedon Jan. 7, 2011, which claims priority to U.S. Provisional ApplicationNos. 61/292,868, 61/328,189, 61/334,555, and 61/345,154 filed on Jan. 7,2010, Apr. 27, 2010, May 13, 2010, and May 17, 2010, and which claimspriority to Patent Application No. 10-2011-0001669 filed in the Republicof Korea on Jan. 7, 2011, the entire contents of all of the aboveapplications are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to wireless communication, and moreparticularly, to a method and apparatus for generating a referencesignal sequence in a wireless communication system.

Discussion of the Related Art

Multiple-input multiple-output (MIMO) technology can be used to improvethe efficiency of data transmission and reception using multipletransmission antennas and multiple reception antennas. MIMO technologymay include a space frequency block code (SFBC), a space time block code(STBC), a cyclic delay diversity (CDD), a frequency switched transmitdiversity (FSTD), a time switched transmit diversity (TSTD), a precodingvector switching (PVS), spatial multiplexing (SM) for implementingdiversity. An MIMO channel matrix according to the number of receptionantennas and the number of transmission antennas can be decomposed intoa number of independent channels. Each of the independent channels iscalled a layer or stream. The number of layers is called a rank.

In wireless communication systems, it is necessary to estimate an uplinkchannel or a downlink channel for the purpose of the transmission andreception of data, the acquisition of system synchronization, and thefeedback of channel information. In wireless communication systemenvironments, fading is generated because of multi-path time latency. Aprocess of restoring a transmit signal by compensating for thedistortion of the signal resulting from a sudden change in theenvironment due to such fading is referred to as channel estimation. Itis also necessary to measure the state of a channel for a cell to whicha user equipment belongs or other cells. To estimate a channel ormeasure the state of a channel, a reference signal (RS) which is knownto both a transmitter and a receiver can be used.

A subcarrier used to transmit the reference signal is referred to as areference signal subcarrier, and a subcarrier used to transmit data isreferred to as a data subcarrier. In an OFDM system, a method ofassigning the reference signal includes a method of assigning thereference signal to all the subcarriers and a method of assigning thereference signal between data subcarriers. The method of assigning thereference signal to all the subcarriers is performed using a signalincluding only the reference signal, such as a preamble signal, in orderto obtain the throughput of channel estimation. If this method is used,the performance of channel estimation can be improved as compared withthe method of assigning the reference signal between data subcarriersbecause the density of reference signals is in general high. However,since the amount of transmitted data is small in the method of assigningthe reference signal to all the subcarriers, the method of assigning thereference signal between data subcarriers is used in order to increasethe amount of transmitted data. If the method of assigning the referencesignal between data subcarriers is used, the performance of channelestimation can be deteriorated because the density of reference signalsis low. Accordingly, the reference signals should be properly arrangedin order to minimize such deterioration.

A receiver can estimate a channel by separating information about areference signal from a received signal because it knows the informationabout a reference signal and can accurately estimate data, transmittedby a transmit stage, by compensating for an estimated channel value.Assuming that the reference signal transmitted by the transmitter is p,channel information experienced by the reference signal duringtransmission is h, thermal noise occurring in the receiver is n, and thesignal received by the receiver is y, it can result in y=h·p+n. Here,since the receiver already knows the reference signal p, it can estimatea channel information value ĥ using Equation 1 in the case in which aLeast Square (LS) method is used.ĥ=y/p=h+n/p=h+{circumflex over (n)}  [Equation 1]

The accuracy of the channel estimation value ĥ estimated using thereference signal p is determined by the value {circumflex over (n)}. Toaccurately estimate the value h, the value {circumflex over (n)} mustconverge on 0. To this end, the influence of the value {circumflex over(n)} has to be minimized by estimating a channel using a large number ofreference signals. A variety of algorithms for a better channelestimation performance may exist.

In order to minimize inter-cell interference (ICI) in transmitting areference signal, sequence group hopping (SGH) or sequence hopping (SH)may be applied to a reference signal sequence. When the SGH is applied,the sequence group index of a reference signal sequence transmitted ineach slot may be changed.

In a multi-user (MU) MIMO environment, in order to guaranteeorthogonality between reference signals transmitted by a plurality ofUEs, an orthogonal covering code (OCC) may be used. When the OCC isapplied, the improvement of orthogonality and throughput can beguaranteed. Meanwhile, in an MU-MIMO environment, a plurality of UEs mayuse different bandwidths. If the OCC is applied while the SCH isperformed on reference signals transmitted by the plurality of UEshaving different bandwidths, the complexity of cell planning isincreased. That is, it is difficult to guarantee orthogonality thereference signals transmitted by the plurality of UEs.

Accordingly, there is a need for another method to indicate whether toperform SGH or SH on a reference signal sequence.

SUMMARY OF THE INVENTION

The present invention provides a method and apparatus for generating areference signal sequence in a wireless communication system.

In an aspect, a method of generating, by a user equipment (UE), areference signal sequence in a wireless communication system isprovided. The method includes receiving a UE-specific sequence grouphopping (SGH) parameter specified to the UE; and generating thereference signal sequence based on a base sequence for every slot,wherein the base sequence is classified according to a sequence-groupnumber and a base sequence number which are determined for every slot bythe UE-specific SGH parameter indicating whether to perform SGH.

The UE-specific SGH parameter may be transmitted through a higher layer.

The reference signal sequence may be a sequence of a demodulationreference signal (DMRS) that uses a physical uplink shared channel(PUSCH) resources and demodulates a signal.

When the UE-specific SGH parameter indicates that SGH is not performed,a sequence-group number of slots within one subframe and a base sequencenumber within a sequence group may be identical with each other.

When the UE-specific SGH parameter indicates that sequence hopping (SH)is not performed, a sequence-group number of slots within one subframeand a base sequence number within a sequence group may be identical witheach other.

When the UE-specific SGH parameter indicates that SGH is not performed,sequence-group numbers of all slots within a frame may be identical witheach other.

The method may further include receiving a cell-specific SGH parameterindicating whether to perform SGH or a cell-specific SH parameterindicating whether to perform SH. When the cell-specific GH parameterindicates that SGH is performed, the UE-specific SGH parameter mayoverride the cell-specific SGH parameter in indicating whether toperform SGH. When the cell-specific SH parameter indicates that SH isperformed, the UE-specific SGH parameter may override the cell-specificSH parameter in indicating whether to perform SH.

The method may further include transmitting the reference signalsequence by mapping the reference signal sequence to a subcarrier.

The reference signal sequence may be generated further based on a cyclicshift.

The base sequence may be based on a Zadoff-Chu (ZC) sequence.

An orthogonal covering code (OCC) may be applied to the reference signalsequence. Whether to apply the OCC may be indicated by an OCC indextransmitted through a higher layer.

In another aspect, an apparatus for generating a reference signalsequence is provided. The apparatus includes a radio frequency (RF) unitconfigured to receive a user equipment (UE)-specific sequence grouphopping (SGH) parameter, and a processor coupled to the RF unit andconfigured to generate the signal sequence based on a base sequence forevery slot, wherein the base sequence is classified according to asequence-group number and a base sequence number which are determinedfor every slot by the UE-specific SGH parameter indicating whether toperform SGH.

In a MU-MIMO environment, orthogonality between a plurality of UEs usingdifferent bandwidths can be guaranteed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a wireless communication system.

FIG. 2 shows the structure of a radio frame in 3GPP LTE.

FIG. 3 shows an example of a resource grid of a single downlink slot.

FIG. 4 shows the structure of a downlink subframe.

FIG. 5 shows the structure of an uplink subframe.

FIG. 6 shows an example of the structure of a transmitter in an SC-FDMAsystem.

FIG. 7 shows an example of a scheme in which the subcarrier mapper mapsthe complex-valued symbols to the respective subcarriers of thefrequency domain.

FIG. 8 shows an example of the structure of a reference signaltransmitter for demodulation.

FIG. 9 shows examples of a subframe through which a reference signal istransmitted.

FIG. 10 shows an example of a transmitter using the clustered DFT-s OFDMtransmission scheme.

FIG. 11 shows another example of a transmitter using the clustered DFT-sOFDM transmission scheme.

FIG. 12 is another example of a transmitter using the clustered DFT-sOFDM transmission scheme.

FIG. 13 shows an example where the OCC is applied to a reference signal.

FIG. 14 is an example where a plurality of UEs performs MU-MIMOtransmission using different bandwidths.

FIG. 15 is an example where SGH and SH are not performed by the proposedUE-specific SGH parameter.

FIG. 16 is an embodiment of a proposed method of generating a referencesignal sequence.

FIG. 17 is a block diagram showing a BS and UE in which the embodimentsof the present invention are implemented.

DETAILED DESCRIPTION OF THE INVENTION

The following technique may be used for various wireless communicationsystems such as code division multiple access (CDMA), a frequencydivision multiple access (FDMA), time division multiple access (TDMA),orthogonal frequency division multiple access (OFDMA), singlecarrier-frequency division multiple access (SC-FDMA), and the like. TheCDMA may be implemented as a radio technology such as universalterrestrial radio access (UTRA) or CDMA2000. The TDMA may be implementedas a radio technology such as a global system for mobile communications(GSM)/general packet radio service (GPRS)/enhanced data rates for GSMevolution (EDGE). The OFDMA may be implemented by a radio technologysuch as institute of electrical and electronics engineers (IEEE) 802.11(Wi-Ei) IEEE 802.16 (WiMAX), IEEE 802.20, E-UTRA (evolved UTRA), and thelike. IEEE 802.16m, an evolution of IEEE 802.16e, provides backwardcompatibility with a system based on IEEE 802.16e. The UTRA is part of auniversal mobile telecommunications system (UMTS). 3GPP (3rd generationpartnership project) LTE (long term evolution) is part of an evolvedUMTS (E-UMTS) using the E-UTRA, which employs the OFDMA in downlink andthe SC-FDMA in uplink. LTE-A (advanced) is an evolution of 3GPP LTE.

Hereinafter, for clarification, LTE-A will be largely described, but thetechnical concept of the present invention is not meant to be limitedthereto.

FIG. 1 shows a wireless communication system.

The wireless communication system 10 includes at least one base station(BS) 11. Respective BSs 11 provide a communication service to particulargeographical areas 15 a, 15 b, and 15 c (which are generally calledcells). Each cell may be divided into a plurality of areas (which arecalled sectors). A user equipment (UE) 12 may be fixed or mobile and maybe referred to by other names such as MS (mobile station), MT (mobileterminal), UT (user terminal), SS (subscriber station), wireless device,PDA (personal digital assistant), wireless modem, handheld device. TheBS 11 generally refers to a fixed station that communicates with the UE12 and may be called by other names such as eNB (evolved-NodeB), BTS(base transceiver system), access point (AP), etc.

In general, a UE belongs to one cell, and the cell to which a UE belongsis called a serving cell. A BS providing a communication service to theserving cell is called a serving BS. The wireless communication systemis a cellular system, so a different cell adjacent to the serving cellexists. The different cell adjacent to the serving cell is called aneighbor cell. A BS providing a communication service to the neighborcell is called a neighbor BS. The serving cell and the neighbor cell arerelatively determined based on a UE.

This technique can be used for downlink or uplink. In general, downlinkrefers to communication from the BS 11 to the UE 12, and uplink refersto communication from the UE 12 to the BS 11. In downlink, a transmittermay be part of the BS 11 and a receiver may be part of the UE 12. Inuplink, a transmitter may be part of the UE 12 and a receiver may bepart of the BS 11.

The wireless communication system may be any one of a multiple-inputmultiple-output (MIMO) system, a multiple-input single-output (MISO)system, a single-input single-output (SISO) system, and a single-inputmultiple-output (SIMO) system. The MIMO system uses a plurality oftransmission antennas and a plurality of reception antennas. The MISOsystem uses a plurality of transmission antennas and a single receptionantenna. The SISO system uses a single transmission antenna and a singlereception antenna. The SIMO system uses a single transmission antennaand a plurality of reception antennas. Hereinafter, a transmissionantenna refers to a physical or logical antenna used for transmitting asignal or a stream, and a reception antenna refers to a physical orlogical antenna used for receiving a signal or a stream.

FIG. 2 shows the structure of a radio frame in 3GPP LTE.

It may be referred to Paragraph 5 of “Technical Specification GroupRadio Access Network; Evolved Universal Terrestrial Radio Access(E-UTRA); Physical channels and modulation (Release 8)” to 3GPP (3rdgeneration partnership project) TS 36.211 V8.2.0 (2008-03). Referring toFIG. 2, the radio frame includes 10 subframes, and one subframe includestwo slots. The slots in the radio frame are numbered by #0 to #19. Atime taken for transmitting one subframe is called a transmission timeinterval (TTI). The TTI may be a scheduling unit for a datatransmission. For example, a radio frame may have a length of 10 ms, asubframe may have a length of 1 ms, and a slot may have a length of 0.5ms.

One slot includes a plurality of orthogonal frequency divisionmultiplexing (OFDM) symbols in a time domain and a plurality ofsubcarriers in a frequency domain. Since 3GPP LTE uses OFDMA indownlink, the OFDM symbols are used to express a symbol period. The OFDMsymbols may be called by other names depending on a multiple-accessscheme. For example, when a single carrier frequency division multipleaccess (SC-FDMA) is in use as an uplink multi-access scheme, the OFDMsymbols may be called SC-FDMA symbols. A resource block (RB), a resourceallocation unit, includes a plurality of continuous subcarriers in aslot. The structure of the radio frame is merely an example. Namely, thenumber of subframes included in a radio frame, the number of slotsincluded in a subframe, or the number of OFDM symbols included in a slotmay vary.

3GPP LTE defines that one slot includes seven OFDM symbols in a normalcyclic prefix (CP) and one slot includes six OFDM symbols in an extendedCP.

The wireless communication system may be divided into a frequencydivision duplex (FDD) scheme and a time division duplex (TDD) scheme.According to the FDD scheme, an uplink transmission and a downlinktransmission are made at different frequency bands. According to the TDDscheme, an uplink transmission and a downlink transmission are madeduring different periods of time at the same frequency band. A channelresponse of the TDD scheme is substantially reciprocal. This means thata downlink channel response and an uplink channel response are almostthe same in a given frequency band. Thus, the TDD-based wirelesscommunication system is advantageous in that the downlink channelresponse can be obtained from the uplink channel response. In the TDDscheme, the entire frequency band is time-divided for uplink anddownlink transmissions, so a downlink transmission by the BS and anuplink transmission by the UE can be simultaneously performed. In a TDDsystem in which an uplink transmission and a downlink transmission arediscriminated in units of subframes, the uplink transmission and thedownlink transmission are performed in different subframes.

FIG. 3 shows an example of a resource grid of a single downlink slot.

A downlink slot includes a plurality of OFDM symbols in the time domainand N_(RB) number of resource blocks (RBs) in the frequency domain. TheN_(RB) number of resource blocks included in the downlink slot isdependent upon a downlink transmission bandwidth set in a cell. Forexample, in an LTE system, N_(KB) may be any one of 60 to 110. Oneresource block includes a plurality of subcarriers in the frequencydomain. An uplink slot may have the same structure as that of thedownlink slot.

Each element on the resource grid is called a resource element. Theresource elements on the resource grid can be discriminated by a pair ofindexes (k,l) in the slot. Here, k (k=0, . . . , N_(RB)×12−1) is asubcarrier index in the frequency domain, and l is an OFDM symbol indexin the time domain.

Here, it is illustrated that one resource block includes 7×12 resourceelements made up of seven OFDM symbols in the time domain and twelvesubcarriers in the frequency domain, but the number of OFDM symbols andthe number of subcarriers in the resource block are not limited thereto.The number of OFDM symbols and the number of subcarriers may varydepending on the length of a cyclic prefix (CP), frequency spacing, andthe like. For example, in case of a normal CP, the number of OFDMsymbols is 7, and in case of an extended CP, the number of OFDM symbolsis 6. One of 128, 256, 512, 1024, 1536, and 2048 may be selectively usedas the number of subcarriers in one OFDM symbol.

FIG. 4 shows the structure of a downlink subframe.

A downlink subframe includes two slots in the time domain, and each ofthe slots includes seven OFDM symbols in the normal CP. First three OFDMsymbols (maximum four OFDM symbols with respect to a 1.4 MHz bandwidth)of a first slot in the subframe corresponds to a control region to whichcontrol channels are allocated, and the other remaining OFDM symbolscorrespond to a data region to which a physical downlink shared channel(PDSCH) is allocated.

The PDCCH may carry a transmission format and a resource allocation of adownlink shared channel (DL-SCH), resource allocation information of anuplink shared channel (UL-SCH), paging information on a PCH, systeminformation on a DL-SCH, a resource allocation of an higher layercontrol message such as a random access response transmitted via aPDSCH, a set of transmission power control commands with respect toindividual UEs in a certain UE group, an activation of a voice overinternet protocol (VoIP), and the like. A plurality of PDCCHs may betransmitted in the control region, and a UE can monitor a plurality ofPDCCHs. The PDCCHs are transmitted on one or an aggregation of aplurality of consecutive control channel elements (CCE). The CCE is alogical allocation unit used to provide a coding rate according to thestate of a wireless channel. The CCE corresponds to a plurality ofresource element groups. The format of the PDCCH and an available numberof bits of the PDCCH are determined according to an associative relationbetween the number of the CCEs and a coding rate provided by the CCEs.

The BS determines a PDCCH format according to a DCI to be transmitted tothe UE, and attaches a cyclic redundancy check (CRC) to the DCI. Aunique radio network temporary identifier (RNTI) is masked on the CRCaccording to the owner or the purpose of the PDCCH. In case of a PDCCHfor a particular UE, a unique identifier, e.g., a cell-RNTI (C-RNTI), ofthe UE, may be masked on the CRC. Or, in case of a PDCCH for a pagingmessage, a paging indication identifier, e.g., a paging-RNTI (P-RNTI),may be masked on the CRC. In case of a PDCCH for a system informationblock (SIB), a system information identifier, e.g., a systeminformation-RNTI (SI-RNTI), may be masked on the CRC. In order toindicate a random access response, i.e., a response to a transmission ofa random access preamble of the UE, a random access-RNTI (RA-RNTI) maybe masked on the CRC.

FIG. 5 shows the structure of an uplink subframe.

An uplink subframe may be divided into a control region and a dataregion in the frequency domain. A physical uplink control channel(PUCCH) for transmitting uplink control information is allocated to thecontrol region. A physical uplink shared channel (PUCCH) fortransmitting data is allocated to the data region. If indicated by ahigher layer, the user equipment may support simultaneous transmissionof the PUCCH and the PUSCH.

The PUCCH for one UE is allocated in an RB pair. RBs belonging to the RBpair occupy different subcarriers in each of a 1^(st) slot and a 2^(nd)slot. A frequency occupied by the RBs belonging to the RB pair allocatedto the PUCCH changes at a slot boundary. This is called that the RB pairallocated to the PUCCH is frequency-hopped at a slot boundary. Since theUE transmits UL control information over time through differentsubcarriers, a frequency diversity gain can be obtained. In the figure,m is a location index indicating a logical frequency-domain location ofthe RB pair allocated to the PUCCH in the subframe.

Uplink control information transmitted on the PUCCH may include a HARQACK/NACK, a channel quality indicator (CQI) indicating the state of adownlink channel, a scheduling request (SR) which is an uplink radioresource allocation request, and the like.

The PUSCH is mapped to a uplink shared channel (UL-SCH), a transportchannel. Uplink data transmitted on the PUSCH may be a transport block,a data block for the UL-SCH transmitted during the TTI. The transportblock may be user information. Or, the uplink data may be multiplexeddata. The multiplexed data may be data obtained by multiplexing thetransport block for the UL-SCH and control information. For example,control information multiplexed to data may include a CQI, a precedingmatrix indicator (PMI), an HARQ, a rank indicator (RI), or the like. Orthe uplink data may include only control information.

FIG. 6 shows an example of the structure of a transmitter in an SC-FDMAsystem.

Referring to FIG. 6, the transmitter 50 includes a discrete Fouriertransform (DPT) unit 51, a subcarrier mapper 52, an inverse fast Fouriertransform (IFFT) unit 53, and a cyclic prefix (CP) insertion unit 54.The transmitter 50 may include a scramble unit (not shown), a modulationmapper (not shown), a layer mapper (not shown), and a layer permutator(not shown), which may be placed in front of the DFT unit 51.

The DFT unit 51 outputs complex-valued symbols by performing DFT oninput symbols. For example, when Ntx symbols are input (where Ntx is anatural number), a DFT size is Ntx. The DFT unit 51 may be called atransform precoder. The subcarrier mapper 52 maps the complex-valuedsymbols to the respective subcarriers of the frequency domain. Thecomplex-valued symbols may be mapped to resource elements correspondingto a resource block allocated for data transmission. The subcarriermapper 52 may be called a resource element mapper. The IFFT unit 53outputs a baseband signal for data (that is, a time domain signal) byperforming IFFT on the input symbols. The CP insertion unit 54 copiessome of the rear part of the baseband signal for data and inserts thecopied parts into the former part of the baseband signal for data.Orthogonality may be maintained even in a multi-path channel becauseinter-symbol interference (ISI) and inter-carrier interference (ICI) areprevented through CP insertion.

FIG. 7 shows an example of a scheme in which the subcarrier mapper mapsthe complex-valued symbols to the respective subcarriers of thefrequency domain. Referring to FIG. 7(a), the subcarrier mapper maps thecomplex-valued symbols, outputted from the DFT unit, to subcarrierscontiguous to each other in the frequency domain. ‘0’ is inserted intosubcarriers to which the complex-valued symbols are not mapped. This iscalled localized mapping. In a 3GPP LTE system, a localized mappingscheme is used. Referring to FIG. 7(b), the subcarrier mapper inserts an(L−1) number of ‘0’ every two contiguous complex-valued symbols whichare outputted from the DFT unit (L is a natural number). That is, thecomplex-valued symbols outputted from the DFT unit are mapped tosubcarriers distributed at equal intervals in the frequency domain. Thisis called distributed mapping. If the subcarrier mapper uses thelocalized mapping scheme as in FIG. 7(a) or the distributed mappingscheme as in FIG. 7(b), a single carrier characteristic is maintained.

FIG. 8 shows an example of the structure of a reference signaltransmitter for demodulation.

Referring to FIG. 8, the reference signal transmitter 60 includes asubcarrier mapper 61, an IFFT unit 62, and a CP insertion unit 63.Unlike the transmitter 50 of FIG. 6, in the reference signal transmitter60, a reference signal is directly generated in the frequency domainwithout passing through the DFT unit 51 and then mapped to subcarriersthrough the subcarrier mapper 61. Here, the subcarrier mapper may mapthe reference signal to the subcarriers using the localized mappingscheme of FIG. 7(a).

FIG. 9 shows examples of a subframe through which a reference signal istransmitted. The structure of a subframe in FIG. 9(a) shows a case of anormal CP. The subframe includes a first slot and a second slot. Each ofthe first slot and the second slot includes 7 OFDM symbols. The 14 OFDMsymbols within the subframe are assigned respective symbol indices 0 to13. Reference signals may be transmitted through the OFDM symbols havingthe symbol indices 3 and 10. The reference signals may be transmittedusing a sequence. A Zadoff-Chu (ZC) sequence may be used as thereference signal sequence. A variety of ZC sequences may be generatedaccording to a root index and a cyclic shift value. A BS may estimatethe channels of a plurality of UEs through an orthogonal sequence or aquasi-orthogonal sequence by allocating different cyclic shift values tothe UEs. The positions of the reference signals occupied in the twoslots within the subframe in the frequency domain may be identical witheach other or different from each other. In the two slots, the samereference signal sequence is used. Data may be transmitted through theremaining SC-FDMA symbols other than the SC-FDMA symbols through whichthe reference signals are transmitted. The structure of a subframe inFIG. 9(b) shows a case of an extended CP. The subframe includes a firstslot and a second slot. Each of the first slot and the second slotincludes 6 SC-FDMA symbols. The 12 SC-FDMA symbols within the subframeare assigned symbol indices 0 to 11. Reference signals are transmittedthrough the SC-FDMA symbols having the symbol indices 2 and 8. Data istransmitted through the remaining SC-FDMA symbols other than the SC-FDMAsymbols through which the reference signals are transmitted.

Although not shown in FIG. 9, a sounding reference signal (SRS) may betransmitted through the OFDM symbols within the subframe. The SRS is areference signal for UL scheduling which is transmitted from UE to a BS.The BS estimates a UL channel through the received SRS and uses theestimated UL channel in UL scheduling.

A clustered DPT-s OFDM transmission scheme is a modification of theexisting SC-FDMA transmission scheme and is a method of dividing datasymbols, subjected to a precoder, into a plurality of subblocks,separating the subblocks, and mapping the subblocks in the frequencydomain.

FIG. 10 shows an example of a transmitter using the clustered DFT-s OFDMtransmission scheme. Referring to FIG. 10, the transmitter 70 includes aDFT unit 71, a subcarrier mapper 72, an IFFT unit 73, and a CP insertionunit 74. The transmitter 70 may further include a scramble unit (notshown), a modulation mapper (not shown), a layer mapper (not shown), anda layer permutator (not shown), which may be placed in front of the DFTunit 71.

Complex-valued symbols outputted from the DFT unit 71 are divided into Nsubblocks (N is a natural number). The N subblocks may be represented bya subblock #1, a subblock #2, . . . , a subblock #N. The subcarriermapper 72 distributes the N subblocks in the frequency domain and mapsthe N subblocks to subcarriers. The NULL may be inserted every twocontiguous subblocks. The complex-valued symbols within one subblock maybe mapped to subcarriers contiguous to each other in the frequencydomain. That is, the localized mapping scheme may be used within onesubblock.

The transmitter 70 of FIG. 10 may be used both in a single carriertransmitter or a multi-carrier transmitter. If the transmitter 70 isused in the single carrier transmitter, all the N subblocks correspondto one carrier. If the transmitter 70 is used in the multi-carriertransmitter, each of the N subblocks may correspond to one carrier.Alternatively, even if the transmitter 70 is used in the multi-carriertransmitter, a plurality of subblocks of the N subblocks may correspondto one carrier. Meanwhile, in the transmitter 70 of FIG. 10, a timedomain signal is generated through one IFFT unit 73. Accordingly, inorder for the transmitter 70 of FIG. 10 to be used in a multi-carriertransmitter, subcarrier intervals between contiguous carriers in acontiguous carrier allocation situation must be aligned.

FIG. 11 shows another example of a transmitter using the clustered DFT-sOFDM transmission scheme. Referring to FIG. 11, the transmitter 80includes a DFT unit 81, a subcarrier mapper 82, a plurality of IFFTunits 83-1, 83-2, . . . , 83-N (N is a natural number), and a CPinsertion unit 84. The transmitter 80 may further include a scrambleunit (not shown), a modulation mapper (not shown), a layer mapper (notshown), and a layer permutator (not shown), which may be placed in frontof the DFT unit 71.

IFFT is individually performed on each of N subblocks. An n^(th) IFFTunit 38-n outputs an n^(th) baseband signal (n=1, 2, . . . , N) byperforming IFFT on a subblock #n. The n^(th) baseband signal ismultiplied by an n^(th) carrier signal to produce an n^(th) radiosignal. After the N radio signals generated from the N subblocks areadded, a CP is inserted by the CP insertion unit 314. The transmitter 80of FIG. 11 may be used in a discontiguous carrier allocation situationwhere carriers allocated to the transmitter are not contiguous to eachother.

FIG. 12 is another example of a transmitter using the clustered DFT-sOFDM transmission scheme. FIG. 12 is a chunk-specific DFT-s OFDM systemperforming DFT precoding on a chunk basis. This may be called NxSC-FDMA. Referring to FIG. 12, the transmitter 90 includes a code blockdivision unit 91, a chunk division unit 92, a plurality of channelcoding units 93-1, . . . , 93-N, a plurality of modulators 94-1, . . . ,4914-N, a plurality of DFT units 95-1, . . . , 95-N, a plurality ofsubcarrier mappers 96-1, . . . , 96-N, a plurality of WIT units 97-1, .. . , 97-N, and a CP insertion unit 98. Here, N may be the number ofmultiple carriers used by a multi-carrier transmitter. Each of thechannel coding units 93-1, . . . , 93-N may include a scramble unit (notshown). The modulators 94-1, . . . , 94-N may also be called modulationmappers. The transmitter 90 may further include a layer mapper (notshown) and a layer permutator (not shown) which may be placed in frontof the DFT units 95-1, . . . , 95-N.

The code block division unit 91 divides a transmission block into aplurality of code blocks. The chunk division unit 92 divides the codeblocks into a plurality of chunks. Here, the code block may be datatransmitted by a multi-carrier transmitter, and the chunk may be a datapiece transmitted through one of multiple carriers. The transmitter 90performs DFT on a chunk basis. The transmitter 90 may be used in adiscontiguous carrier allocation situation or a contiguous carrierallocation situation.

A UL reference signal is described below.

In general, the reference signal is transmitted in the form of asequence. A specific sequence may be used as the reference signalsequence without a special limit. A phase shift keying (PSK)-basedcomputer generated sequence may be used as the reference signalsequence. Examples of PSK include binary phase shift keying (BPSK) andquadrature phase shift keying (QPSK). Alternatively, a constantamplitude zero auto-correlation (CAZAC) sequence may be used as thereference signal sequence. Examples of the CAZAC sequence include aZadoff-Chu (ZC)-based sequence, a ZC sequence with cyclic extension, anda ZC sequence with truncation. Alternatively, a pseudo-random (PN)sequence may be used as the reference signal sequence. Examples of thePN sequence include an m-sequence, a computer-generated sequence, a goldsequence, and a Kasami sequence. A cyclically shifted sequence may beused as the reference signal sequence.

A UL reference signal may be divided into a demodulation referencesignal (DMRS) and a sounding reference signal (SRS). The DMRS is areference signal used in channel estimation for the demodulation of areceived signal. The DMRS may be associated with the transmission of aPUSCH or PUCCH. The SRS is a reference signal transmitted from a UE to aBS for UL scheduling. The BS estimates an UL channel through thereceived SRS and uses the estimated UL channel in UL scheduling. The SRSis not associated with the transmission of a PUSCH or PUCCH. The samekind of a basic sequence may be used for the DMRS and the SRS.Meanwhile, in UL multi-antenna transmission, preceding applied to theDMRS may be the same as preceding applied to a PUSCH. Cyclic shiftseparation is a primary scheme for multiplexing the DMRS. In an LTE-Asystem, the SRS may not be precoded and may be an antenna-specificreference signal.

A reference signal sequence r_(u,v) ^((α))(n) may be defined based on abasic sequence b_(u,v)(n) and a cyclic shift a according to Equation 2.r _(u,v) ^((α))(n)=e ^(jαn) b _(u,v)(n),0≦n<M _(sc) ^(RS)  [Equation 2]

In Equation 2, M_(sc) ^(RS)(1≦m≦N_(RB) ^(max,UL)) is the length of thereference signal sequence and M_(sc) ^(RS)=m*N_(sc) ^(RB). N_(sc) ^(RB)is the size of a resource block, indicated by the number of subcarriersin the frequency domain. N_(RB) ^(max,UL) indicates a maximum value of aUL bandwidth indicated by a multiple of N_(sc) ^(RB). A plurality ofreference signal sequences may be defined by differently applying acyclic shift value a from one basic sequence.

A basic sequence b_(u,v)(n) is divided into a plurality of groups. Here,uε{0, 1, . . . , 29} indicates a group index, and v indicates a basicsequence index within the group. The basic sequence depends on thelength M_(sc) ^(RS) of the basic sequence. Each group includes a basicsequence (v=0) having a length of M_(sc) ^(RS) for m (6≦m≦n_(RB)^(max,UL)) and includes 2 basic sequences (v=0,1) having a length ofM_(sc) ^(RS) for m (6≦m≦n_(RB) ^(max,UL)). The sequence group index uand the basic sequence index v within a group may vary according to timeas in group hopping or sequence hopping.

Furthermore, if the length of the reference signal sequence is 3N_(sc)^(RB) or higher, the basic sequence may be defined by Equation 3.b _(u,v)(n)=x _(q)(n mod N _(ZC) ^(RS)),0≦n<M _(sc) ^(RS)  [Equation 3]

In Equation 3, q indicates a root index of a Zadoff-Chu (ZC) sequence.N_(ZC) ^(RS) is the length of the ZC sequence and may be a maximum primenumber smaller than M_(sc) ^(RS). The ZC sequence having the root indexq may be defined by Equation 4.

$\begin{matrix}{{{x_{q}(m)} = {\mathbb{e}}^{{- j}\frac{\pi\;{qm}{({m + 1})}}{N_{ZC}^{RS}}}},{0 \leq m \leq {N_{ZC}^{RS} - 1}}} & \left\lbrack {{Equation}\mspace{14mu} 4} \right\rbrack\end{matrix}$

q may be given by Equation 5.q=└q+½┘+v·(−1)^(└2q┘)q=N _(ZC) ^(RS)·(u+1)/31  [Equation 5]

If the length of the reference signal sequence is 3N_(sc) ^(RB) or less,the basic sequence may be defined by Equation 6.b _(u,v)(n)=e ^(jφ(n)π/4),0≦n≦M _(sc) ^(RS)−1  [Equation 6]

Table 1 is an example where φ(n) is defined when MscRS=NscRB.

TABLE 1 φ(0), . . . , φ(11) 0 −1 1 3 −3 3 3 1 1 3 1 −3 3 1 1 1 3 3 3 −11 −3 −3 1 −3 3 2 1 1 −3 −3 −3 −1 −3 −3 1 −3 1 −1 3 −1 1 1 1 1 −1 −3 −3 1−3 3 −1 4 −1 3 1 −1 1 −1 −3 −1 1 −1 1 3 5 1 −3 3 −1 −1 1 1 −1 −1 3 −3 16 −1 3 −3 −3 −3 3 1 −1 3 3 −3 1 7 −3 −1 −1 −1 1 −3 3 −1 1 −3 3 1 8 1 −33 1 −1 −1 −1 1 1 3 −1 1 9 1 −3 −1 3 3 −1 −3 1 1 1 1 1 10 −1 3 −1 1 1 −3−3 −1 −3 −3 3 −1 11 3 1 −1 −1 3 3 −3 1 3 1 3 3 12 1 −3 1 1 −3 1 1 1 −3−3 −3 1 13 3 3 −3 3 −3 1 1 3 −1 −3 3 3 14 −3 1 −1 −3 −1 3 1 3 3 3 −1 115 3 −1 1 −3 −1 −1 1 1 3 1 −1 −3 16 1 3 1 −1 1 3 3 3 −1 −1 3 −1 17 −3 11 3 −3 3 −3 −3 3 1 3 −1 18 −3 3 1 1 −3 1 −3 −3 −1 −1 1 −3 19 −1 3 1 3 1−1 −1 3 −3 −1 −3 −1 20 −1 −3 1 1 1 1 3 1 −1 1 −3 −1 21 −1 3 −1 1 −3 −3−3 −3 −3 1 −1 −3 22 1 1 −3 −3 −3 −3 −1 3 −3 1 −3 3 23 1 1 −1 −3 −1 −3 1−1 1 3 −1 1 24 1 1 3 1 3 3 −1 1 −1 −3 −3 1 25 1 −3 3 3 1 3 3 1 −3 −1 −13 26 1 3 −3 −3 3 −3 1 −1 −1 3 −1 −3 27 −3 −1 −3 −1 −3 3 1 −1 1 3 −3 −328 −1 3 −3 3 −1 3 3 −3 3 3 −1 −1 29 3 −3 −3 −1 −1 −3 −1 3 −3 3 1 −1

Table 2 is an example where φ(n) is defined when M_(sc) ^(RS)=2*N_(sc)^(RB).

TABLE 2 φ(0), . . . , φ(23) 0 −1 3 1 −3 3 −1 1 3 −3 3 1 3 −3 3 1 1 −1 13 −3 3 −3 −1 −3 1 −3 3 −3 −3 −3 1 −3 −3 3 −1 1 1 1 3 1 −1 3 −3 −3 1 3 11 −3 2 3 −1 3 3 1 1 −3 3 3 3 3 1 −1 3 −1 1 1 −1 −3 −1 −1 1 3 3 3 −1 −3 11 3 −3 1 1 −3 −1 −1 1 3 1 3 1 −1 3 1 1 −3 −1 −3 −1 4 −1 −1 −1 −3 −3 −1 11 3 3 −1 3 −1 1 −1 −3 1 −1 −3 −3 1 −3 −1 −1 5 −3 1 1 3 −1 1 3 1 −3 1 −31 1 −1 −1 3 −1 −3 3 −3 −3 −3 1 1 6 1 1 −1 −1 3 −3 −3 3 −3 1 −1 −1 1 −1 11 −1 −3 −1 1 −1 3 −1 −3 7 −3 3 3 −1 −1 −3 −1 3 1 3 1 3 1 1 −1 3 1 −1 1 3−3 −1 −1 1 8 −3 1 3 −3 1 −1 −3 3 −3 3 −1 −1 −1 −1 1 −3 −3 −3 1 −3 −3 −31 −3 9 1 1 −3 3 3 −1 −3 −1 3 −3 3 3 3 −1 1 1 −3 1 −1 1 1 −3 1 1 10 −1 1−3 −3 3 −1 3 −1 −1 −3 −3 −3 −1 −3 −3 1 −1 1 3 3 −1 1 −1 3 11 1 3 3 −3 −31 3 1 −1 −3 −3 −3 3 3 −3 3 3 −1 −3 3 −1 1 −3 1 12 1 3 3 1 1 1 −1 −1 1 −33 −1 1 1 −3 3 3 −1 −3 3 −3 −1 −3 −1 13 3 −1 −1 −1 −1 −3 −1 3 3 1 −1 1 33 3 −1 1 1 −3 1 3 −1 −3 3 14 −3 −3 3 1 3 1 −3 3 1 3 1 1 3 3 −1 −1 −3 1−3 −1 3 1 1 3 15 −1 −1 1 −3 1 3 −3 1 −1 −3 −1 3 1 3 1 −1 −3 −3 −1 −1 −3−3 −3 −1 16 −1 −3 3 −1 −1 −1 −1 1 1 −3 3 1 3 3 1 −1 1 −3 1 −3 1 1 −3 −117 1 3 −1 3 3 −1 −3 1 −1 −3 3 3 3 −1 1 1 3 −1 −3 −1 3 −1 −1 −1 18 1 1 11 1 −1 3 −1 −3 1 1 3 −3 1 −3 −1 1 1 −3 −3 3 1 1 −3 19 1 3 3 1 −1 −3 3 −13 3 3 −3 1 −1 1 −1 −3 −1 1 3 −1 3 −3 −3 20 −1 −3 3 −3 −3 −3 −1 −1 −3 −1−3 3 1 3 −3 −1 3 −1 1 −1 3 −3 1 −1 21 −3 −3 1 1 −1 1 −1 1 −1 3 1 −3 −1 1−1 1 −1 −1 3 3 −3 −1 1 −3 22 −3 −1 −3 3 1 −1 −3 −1 −3 −3 3 −3 3 −3 −1 13 1 −3 1 3 3 −1 −3 23 −1 −1 −1 −1 3 3 3 1 3 3 −3 1 3 −1 3 −1 3 3 −3 3 1−1 3 3 24 1 −1 3 3 −1 −3 3 −3 −1 −1 3 −1 3 −1 −1 1 1 1 1 −1 −1 −3 −1 325 1 −1 1 −1 3 −1 3 1 1 −1 −1 −3 1 1 −3 1 3 −3 1 1 −3 −3 −1 −1 26 −3 −11 3 1 1 −3 −1 −1 −3 3 −3 3 1 −3 3 −3 1 −1 1 −3 1 1 1 27 −1 −3 3 3 1 1 3−1 −3 −1 −1 −1 3 1 −3 −3 −1 3 −3 −1 −3 −1 −3 −1 28 −1 −3 −1 −1 1 −3 −1−1 1 −1 −3 1 1 −3 1 −3 −3 3 1 1 −1 3 −1 −1 29 1 1 −1 −1 −3 −1 3 −1 3 −11 3 1 −1 3 1 3 −3 −3 1 −1 −1 1 3

Hopping of a reference signal may be applied as follows.

The sequence group index u of a slot index n_(s) may be defined based ona group hopping pattern f_(gh)(n_(s)) and a sequence shift patternf_(ss) according to Equation 7.u=(f _(gh)(n _(s))+f _(ss))mod 30  [Equation 7]

17 different group hopping patterns and 30 different sequence shiftpatterns may exist. Whether to apply group hopping may be indicated by ahigher layer.

A PUCCH and a PUSCH may have the same group hopping pattern. A grouphopping pattern f_(gh)(n_(s)) may be defined by Equation 8.

$\begin{matrix}{{f_{gh}\left( n_{s} \right)} = \left\{ \begin{matrix}0 & {{if}\mspace{14mu}{group}\mspace{14mu}{hopping}\mspace{14mu}{is}\mspace{14mu}{disabled}} \\{\left( {\sum\limits_{i = 0}^{7}{{c\left( {{8n_{s}} + i} \right)} \cdot 2^{i}}} \right){mod}\; 30} & {{if}\mspace{14mu}{group}\mspace{14mu}{hopping}\mspace{14mu}{enabled}}\end{matrix} \right.} & \left\lbrack {{Equation}\mspace{14mu} 8} \right\rbrack\end{matrix}$

In Equation 8, c(i) is a pseudo random sequence that is a PN sequenceand may be defined by a Gold sequence of a length-31. Equation 9 showsan example of a gold sequence e(n).c(n)=(x ₁(n+N _(c))+x ₂(n+N _(c)))mod 2x ₁(n+31)=(x ₁(n+3)+x ₁(n))mod 2x ₂(n+31)=(x ₂(n+3)+x ₂(n+2)+x ₁(n+1)+x ₁(n))mod 2  [Equation 9]

Here, Nc=1600, x₁(i) is a first m-sequence, and x₂(i) is a secondm-sequence. For example, the first m-sequence or the second m-sequencemay be initialized according to a cell identifier (ID) for every OFDMsymbol, a slot number within one radio frame, an OFDM symbol indexwithin a slot, and the type of a CP. A pseudo random sequence generatormay be initialized to

$c_{init} = \left\lfloor \frac{N_{ID}^{cell}}{30} \right\rfloor$in the first of each radio frame.

A PUCCH and a PUSCH may have the same sequence shift pattern. Thesequence shift pattern of the PUCCH may be f_(ss) ^(PUCCH)=N_(ID)^(cell) mod 30. The sequence shift pattern of the PUSCH may be f_(ss)^(PUSCH)=(f_(ss) ^(PUCCH)+Δ_(ss)) mod 30 and Δ_(ss)ε{0, 1, . . . , 29}may be configured by a higher layer.

Sequence hopping may be applied to only a reference signal sequencehaving a length longer than 6N_(sc) ^(RB). Here, a basic sequence indexv within a basic sequence group of a slot index n may be defined byEquation 10.

$\begin{matrix}{v = \left\{ \begin{matrix}{c\left( n_{s} \right)} & {{if}\mspace{14mu}{group}\mspace{14mu}{hopping}\mspace{14mu}{is}\mspace{14mu}{disabled}\mspace{14mu}{and}} \\\; & {{sequence}\mspace{14mu}{hopping}\mspace{14mu}{is}\mspace{14mu}{enabled}} \\0 & {otherwise}\end{matrix} \right.} & \left\lbrack {{Equation}\mspace{14mu} 10} \right\rbrack\end{matrix}$

c(i) may be represented by an example of Equation 9. Whether to applysequence hopping may be indicated by a higher layer. A pseudo randomsequence generator may be initialized to

$c_{init} = {{\left\lfloor \frac{N_{ID}^{cell}}{30} \right\rfloor \cdot 2^{5}} + f_{ss}^{PUSCH}}$in the first of each radio frame.

A DMRS sequence for a PUSCH may be defined by Equation 11.r ^(PUSCH)(m·M _(sc) ^(RS) +n)=r _(u,v) ^((α))(n)  [Equation 11]

In Equation 11, m=0, 1, . . . and n=0, . . . , M_(sc) ^(RS)−1. M_(sc)^(RS)=M_(sc) ^(PUSCH).

α=2πn_(cs)/12, that is, a cyclic shift value is given within a slot, andn_(cs) may be defined by Equation 12.n _(cs)=(n _(DMRS) ⁽¹⁾ +n _(DMRS) ⁽²⁾ +n _(PRS)(n _(s)))mod12  [Equation 12]

In Equation 12, n_(DMRS) ⁽¹⁾ is indicated by a parameter transmitted bya higher layer, and Table 3 shows an example of a correspondingrelationship between the parameter and n_(DMRS) ⁽¹⁾.

TABLE 3 Parameter n_(DMRS) ⁽¹⁾ 0 0 1 2 2 3 3 4 4 6 5 8 6 9 7 10

Back in Equation 12, n_(DMRS) ⁽²⁾ may be defined by a cyclic shift fieldwithin a DCI format 0 for a transmission block corresponding to PUSCHtransmission. The DCI format is transmitted in a PDCCH. The cyclic shiftfield may have a length of 3 bits.

Table 4 shows an example of a corresponding relationship between thecyclic shift field and n_(DMRS) ⁽²⁾.

TABLE 4 Cyclic shift field in DCI format 0 n_(DMRS) ⁽²⁾ 000 0 001 6 0103 011 4 100 2 101 8 110 10 111 9

If a PDCCH including the DCI format 0 is not transmitted in the sametransmission block, if the first PUSCH is semi-persistently scheduled inthe same transmission block, or if the first PUSCH is scheduled by arandom access response grant in the same transmission block, n_(DMRS)⁽²⁾ may be 0.

n_(PRS)(n_(s)) may be defined by Equation 13.n _(PRS)(n _(s))=Σ_(i=0) ⁷ c(8N _(symb) ^(UL) ·n _(s)+i)·2^(i)  [Equation 13]

c(i) may be represented by the example of Equation 9 and may be appliedin a cell-specific way of c(i). A pseudo random sequence generator maybe initialized to

$c_{init} = {{\left\lfloor \frac{N_{ID}^{cell}}{30} \right\rfloor \cdot 2^{5}} + f_{ss}^{PUSCH}}$in the first of each radio frame.

A DMRS sequence r^(PUSCH) is multiplied by an amplitude scaling factorβ_(PUSCH) and mapped to a physical transmission block, used in relevantPUSCH transmission, from r^(PUSCH)(0) in a sequence starting. The MARSsequence is mapped to a fourth OFDM symbol (OFDM symbol index 3) in caseof a normal CP within one slot and mapped to a third OFDM symbol (OFDMsymbol index 2) within one slot in case of an extended CP.

An SRS sequence r_(SRS)(n)=r_(u,v) ^((α))(n) is defined. u indicates aPUCCH sequence group index, and v indicates a basic sequence index. Thecyclic shift value a is defined by Equation 14.

$\begin{matrix}{\alpha = {2\pi\frac{n_{SRS}^{cs}}{8}}} & \left\lbrack {{Equation}\mspace{14mu} 14} \right\rbrack\end{matrix}$

n_(SRS) ^(cs) is a value configured by a higher layer in related to eachUE and may be any one of integers from 0 to 7.

Meanwhile, an orthogonal code cover (OCC) may be applied to a referencesignal sequence. The OCC means a code which has different orthogonalityand may apply to a sequence. In general, in order to distinguish aplurality of channels from each other, different sequences may be used,but the plurality of channels may be distinguished from each other usingthe OCC.

The OCC may be used for the following purposes.

1) The OCC may be applied in order to increase the amount of radioresources allocated to an uplink reference signal.

For example, assuming that the cyclic shift values of reference signalstransmitted in a first slot and a second slot are allocated as a, a sign(−) may be allocated to the reference signal transmitted in the secondslot. That is, a first user may send a reference signal, having a cyclicshift value of a and a sign (+), in the second slot, and a second usermay send a reference signal, having a cyclic shift value of a and a sign(−), in the second slot. A BS may estimate the channel of the first userby adding the reference signal transmitted in the second slot and thereference signal transmitted in the first slot. Furthermore, the BS mayestimate the channel of the second user by subtracting the referencesignal transmitted in the second slot from the reference signaltransmitted in the first slot. That is, if the OCC is applied, the BScan distinguish the reference signal transmitted by the first user fromthe reference signal transmitted by the second user. Accordingly, theamount of radio resources can be doubled because at least two users usedifferent OCCs while using the same reference signal sequence.

2) The OCC may be applied in order to increase an interval betweencyclic shift values allocated to the multiple antennas or the multiplelayers of a single user. Cyclic shift values allocated to multiplelayers are described below, but cyclic shift values allocated tomultiple antennas may also be applied.

A uplink reference signal distinguishes channels from each other basedon cyclic shift values. In order to distinguish a plurality of layersfrom each other in a multi-antenna system, different cyclic shift valuesmay be allocated to reference signals for respective layers. The numberof cyclic shift values to be allocated must be increased according to anincrease of the number of layers, and thus an interval between thecyclic shift values is reduced. Accordingly, channel estimationperformance is reduced because it is difficult to distinguish aplurality of channels from each other. In order to overcome thisproblem, the OCC may be applied to each layer. For example, it isassumed that cyclic shift offsets 0, 6, 3, and 9 are allocated to therespective reference signals of four layers. An interval between thecyclic shift values of the reference signals of the respective layers is3. Here, the interval between the cyclic shift values of the referencesignals of the layers of antennas may be increased to 6 by applying anOCC of a sign (−) to a third layer and a fourth layer. Accordingly, theperformance of channel estimation can be increased.

3) The OCC may be applied in order to increase an interval betweencyclic shift values allocated to a single user.

In an MU-MIMO system including a plurality of users having multipleantennas, the OCC may be applied to a cyclic shift value. For example,from a viewpoint of a single user performing MIMO transmission, in orderto distinguish a plurality of antennas or a plurality of layers fromeach other, a cyclic shift value having a distance interval betweenantennas or layers may be applied. From a viewpoint of multiple users,however, the cyclic shift interval between the users may be narrowed. Inorder to overcome this problem, the OCC may be used. When the OCC isapplied, the same cyclic shift value may be applied between the multipleusers according to a type of the OCC.

FIG. 13 shows an example where the OCC is applied to a reference signal.

Both a reference signal sequence for a layer 0 and a reference signalsequence for a layer 1 within one subframe are mapped to the fourthSC-FDMA symbol of a first slot and the fourth SC-FDMA symbol of a secondslot. The same sequence is mapped to two SC-FDMA symbols in each layer.Here, the reference signal sequence for the layer 0 is multiplied by anorthogonal sequence [+1 +1] and then mapped to the SC-FDMA symbol. Thereference signal sequence for the layer 1 is multiplied by an orthogonalsequence [++1 −1] and then mapped to the SC-FDMA symbol. That is, whenthe reference signal sequence for the layer 1 is mapped to the secondslot within the one subframe, the reference signal sequence ismultiplied by −1 and then mapped.

If the OCC is applied as described above, a BS that receives a referencesignal may estimate the channel of the layer 0 by adding a referencesignal sequence transmitted in the first slot and a reference signalsequence transmitted in the second slot. Furthermore, the BS mayestimate the channel of the layer 1 by subtracting the reference signalsequence transmitted in the second slot from the reference signalsequence transmitted in the first slot. That is, a BS can distinguishreference signals, transmitted in respective layers, from each other byapplying the OCC. Accordingly, a plurality of reference signals can betransmitted using the same resources. If the number of possible cyclicshift values is 6, the number of layers or users that may be multiplexedusing the OCC can be increased up to 12.

In this example, it is assumed that the binary format [+1 +1] or [+1 or−1] is used as the OCC, but not limited thereto and various kinds oforthogonal sequences may be used as the OCC. For example, orthogonalsequences, such as Walsh codes, DFT coefficients, and CAZAC sequences,may be applied to the OCC. Furthermore, reference signals can bemultiplexed more easily between users having different bandwidths byapplying the OCC.

A proposed method of generating a reference signal sequence is describedbelow.

As described above, whether to perform sequence group hopping (SGH) on areference signal sequence in LTE rel-8 may be indicated by a signal thatis transmitted in a cell-specific way. The cell-specific signalindicating whether to perform SGH on a reference signal sequence ishereinafter called a cell-specific GH parameter. Although LTE rel-8 UEand LTE-A UE coexist within a cell, whether to perform SGH on areference signal sequence is the same in the LTE Rel-8 UE and the LTE-AUE. Currently defined SGH or sequence gopping (SH) may be performed forevery slot. The cell-specific GH parameter may be agroup-hopping-enabled parameter provided by a higher layer. When thevalue of the group-hopping-enabled parameter is true, SGH for areference signal sequence is performed, but SH is not performed. Whenthe value of the group-hopping-enabled parameter is false, SGH for areference signal sequence is not performed, and whether to perform SH isdetermined by a cell-specific SH parameter, provided by a higher layerand indicating whether to perform SH. The cell-specific SH parameter maybe a sequence-hopping-enabled parameter provided by a higher layer.

Meanwhile, in LTE-A, LTE rel-8 UE and LTE-A UE may perform MU-MIMOtransmission, or LTE-A UEs may perform MU-MIMO transmission. Here, inorder to support the MU-MIMO transmission of UEs having differentbandwidths, the OCC may be applied. When the OCC is applied,orthogonality between the UEs performing the MU-MIMO transmission can beimproved and the throughput can also be improved. However, if UEs havedifferent bandwidths and whether to perform SGH or SH for a referencesignal sequence is determined by a cell-specific GH or SH parameterdefined in LTE rel-8, orthogonality between reference signalstransmitted by the respective UEs may not be sufficiently guaranteed.

FIG. 14 is an example where a plurality of UEs performs MU-MIMOtransmission using different bandwidths. In FIG. 14(a), a first UE UE1and a second UE UE2 perform the same bandwidth. In this case, whether toperform SGH or SH for the base sequence of a reference signal may bedetermined by a cell-specific GH or SH parameter defined in LTE rel-8.In FIG. 14(b), a first UE UE1 uses a bandwidth which is the sum ofbandwidths used by a second UE UE2 and a third UE UE3. That is, thefirst UE, the second UE, and the third UE use different bandwidths. Inthis case, whether to perform SGH or SH for the base sequence of areference signal transmitted by each UE needs to be determined using anew method.

Accordingly, a UE-specific SGH parameter may be newly defined inaddition to the existing cell-specific GH parameter and the existingcell-specific SH parameter. The UE-specific SGH parameter is informationfor specific UE and may be transmitted to only the specific UE. TheUE-specific SGH parameter may be applied to a DMRS transmitted usingPUSCH resources allocated to specific UE. That is, the UE-specific SGHparameter may indicate whether to perform SGH/SH for the base sequenceof a DMRS that is transmitted using PUSCH resources. For convenience ofdescription hereinafter, only an example where whether to perform SGHand SH for the base sequence of a reference signal is determined by theUE-specific SGH parameter is described, but not limited thereto. Whetherto apply SH for the base sequence of the reference signal may bedetermined by a UE-specific SH parameter different from the UE-specificSGH parameter. Furthermore, an example where the present invention isapplied to the base sequence of a DMRS transmitted using PUSCH resourcesis described, but not limited thereto. The present invention may also beapplied to a DMRS, an SRS, etc. which are transmitted using PUCCHresources in various ways. Furthermore, an MU-MIMO environment in whicha plurality of UEs has different bandwidths is assumed, but the presentinvention may be applied to an MU-MIMO or SU-MIMO environment in which aplurality of UEs has the same bandwidth.

When a value of the cell-specific GH parameter or the cell-specific SHparameter is true and thus SGH or SH is performed on the base sequenceof a reference signal, SGH or SH of a slot level is in common performedon a DMRS using PUSCH resources and a DMRS and STS using PUCCHresources. That is, the sequence group index (or number) of the basesequence of the reference signal is changed for every slot, or a basesequence index (or number) is changed within a sequence group. Here,whether SGH or SH will be performed on the DMRS using PUSCH resourcesmay be indicated by a UE-specific SGH parameter again. In other words,the UE-specific SGH parameter overrides the cell-specific GH parameteror the cell-specific SGH parameter. The UE-specific SGH parameter may bea disable sequence-group hopping parameter. That is, if a value of theUE-specific SGH parameter is true, SGH and SH may not be performedirrespective of the cell-specific GH parameter or the cell-specific SHparameter. More particularly, when a value of the UE-specific SCHparameter is true, SGH and SH for the base sequence of a referencesignal may not be performed although to execute SGH or SH for the basesequence of the reference signal is indicated by the cell-specific GHparameter or the cell-specific SR parameter. If SGH is not performed, asequence group index of the base sequence of the reference signal maynot be changed for every slot. Furthermore, as in the case where SGH isperformed by the cell-specific GH parameter, a base sequence index ofthe base sequence of the reference signal is not changed for every slotbecause SH is not performed. Here, two slots within a subframe send thebase sequences of the reference signals of base sequence indices, suchas the same sequence group index, because SGH and SH are not performedonly within one subframe, but SGH or SH may be applied betweensubframes. Alternatively, since SGH and SH are not applied within allsubframes, all slots may send the base sequences of the referencesignals of the same sequence group index and the same base sequenceindex. Meanwhile, when a value of the UE-specific SGH parameter isfalse, SGH or SH for the base sequence of a reference signal may beperformed according to the cell-specific GH parameter or thecell-specific SH parameter.

FIG. 15 is an example where SGH and SH are not performed by the proposedUE-specific SGH parameter. Referring to FIG. 15, when SGH and SH areperformed in LTE rel-8 or 9, a sequence group index or a base sequenceindex of base sequence of a reference signal transmitted in each slot isdifferent. Approach 1 is a case where SGH and SH are not performedwithin a subframe according to a UE-specific SGH parameter. Two slotswithin each subframe generate the base sequences of the referencesignals having the same sequence group index and the same base sequenceindex, and a sequence group index or a base sequence index is changedbetween the subframes. Approach 2 is a case where SGH and SH are notperformed within all subframes according to a UE-specific SGH parameter.Accordingly, all the subframes generate the base sequences of thereference signals having the same sequence group index and the same basesequence index.

FIG. 16 is an embodiment of a proposed method of generating a referencesignal sequence.

At step S100, UE receives a UE-specific SGH parameter. The UE-specificSGH parameter may be given by a higher layer. At step S110, the UEgenerates a reference signal sequence based on a base sequence for everyslot. The base sequence may be classified according to a sequence-groupnumber and a base sequence number which are determined for every slot bythe UE-specific SGH parameter indicating whether to perform SGH and SH.

The UE may be informed of whether to perform SGH and SH according to theUE-specific SGH parameter using various methods described below.

1) A frequency hopping flag included in a DCI format for uplinktransmission may play the role of the UE-specific SGH parameter. Forexample, if frequency hopping is enabled by the frequency hopping flag,SGH or SH of a slot level may be performed. Furthermore, if thefrequency hopping is disabled by the frequency hopping flag, SGH and SHfor the base sequence of a DMRS using PUSCH resources may not beperformed. Alternatively, SGH or SH may be performed for every subframe.

2) Whether to perform SGH and SH may be indicated by maskinginformation, indicating whether to perform SGH and SH for the basesequence of a reference signal, in a bit indicating a UE ID included ina DCI format for uplink transmission.

3) Whether to perform SGH and SH for the base sequence of a referencesignal may be indicated when a specific index of a cyclic shiftindicator included in a DCI format for UL transmission is designated.

4) A UE-specific SGH parameter indicating whether to perform SGH and SHfor the base sequence of a reference signal may be included in a DCIformat for UL transmission.

5) A UE-specific SGH parameter may be transmitted to specific UE throughhigher layer signaling for the specific UE.

6) If a clustered DFT-s OFDM transmission scheme is used, SGH and SH forthe base sequence of a reference signal may not be performed.

Meanwhile, when SGH and SH for the base sequence of a reference signalare not performed according to a UE-specific SGH parameter, an OCC maybe applied to the relevant reference signal. If SGH or SH for the basesequence of the reference signal is performed, the OCC may not beapplied.

A variety of methods may be used in order to indicate whether to applythe OCC. First, when a cyclic shift index is indicated through a DCIformat and an OCC index indicating whether to apply an OCC istransmitted through a higher layer, if SGH and SH for the base sequenceof a reference signal are not performed, whether to apply an OCCaccording to an OCC index may be used without change. For example, anOCC may not be applied if an OCC index is 0, and an OCC may be appliedif an OCC index is 1. Alternatively, an OCC may not be applied if an OCCindex is 1, and an OCC may be applied if an OCC index is 0. Furthermore,if SGH or SH for the base sequence of a reference signal is preformed,whether to apply an OCC may be determined in an opposite way to the OCCindex. For example, an OCC may be applied if an OCC index is 0, and anOCC may not be applied if an OCC index is 1. Alternatively, an OCC maybe applied if an DCC index is 1, and an OCC may not be applied if an OCCindex is 0.

Alternatively, an OCC index indicating whether to apply an OCC may notbe separately defined, but a specific OCC may be indicated so that thespecific OCC is applied to specific cyclic shift index by combining acyclic shift index and an OCC index of 3 bits within a DCI format. Here,if SGH for the base sequence of a reference signal is performed, an OCCindex indicated by a relevant cyclic shift index may be reversed again,so that the OCC is not applied. Furthermore, if SGH and SH for the basesequence of a reference signal is not performed according to aUE-specific SGH parameter, the OCC may be applied by using an OCC indexindicated by a relevant cyclic shift index without change. Accordingly,interference between reference signals allocated to respective layerscan be reduced.

In the above description, an example where whether to perform SGH and SHfor the base sequence of a reference signal is determined by aUE-specific SGH parameter has been described. In an MU-MIMO environment,however, in order to further guarantee orthogonality between thereference signals of UEs, a new parameter indicating whether to performSH may be further defined. The new parameter indicating whether toperform SH may be a UE-specific SH parameter. The UE-specific SHparameter may be applied using the same method as the UE-specific SGHparameter. That is, the UE-specific SH parameter may override acell-specific SH parameter. The above-described UE-specific SGHparameter may determine only whether to perform SGH. That is, when avalue of the UE-specific SGH parameter is true, SGH for the basesequence of a reference signal is not performed. Furthermore, whether toperform SH for the base sequence of the reference signal is determinedby the UE-specific SH parameter. When a value of the UE-specific SHparameter is true, SH for the base sequence of the reference signal isnot performed. When a value of the UE-specific SH parameter is false,whether to perform SH for the base sequence of the reference signal maybe determined by a cell-specific SH parameter. The UE-specific SHparameter may be dynamically signalized implicitly or explicitly usingsignaling through a PDCCH or may be given by a higher layer, such as RRCsignaling, implicitly or explicitly.

Meanwhile, in the above description, it has been described that theUE-specific SGH parameter, the UE-specific GH parameter, or theUE-specific SH parameter override the cell-specific GH parameter or thecell-specific SH parameter, irrespective of a UL transmission mode, butmay be changed according to a transmission mode. In LTE rel-8/9, asingle antenna transmission mode is basically supported. In LTE-A,however, a multi-antenna transmission mode, a transmission mode fordiscontinuous allocation, etc. may be defined for the efficiency of ULtransmission. Here, whether to perform the UE-specific SGH parameter,the UE-specific GH parameter, or the UE-specific SH parameter may bedetermined according to the transmission mode. For example, in thesingle antenna transmission mode, although the UE-specific SGH parameteroverrides the cell-specific GH parameter or the cell-specific SHparameter, whether to perform SGH or SH for the base sequence of areference signal may be determined by the cell-specific GH parameter orthe cell-specific SH parameter.

FIG. 17 is a block diagram showing a BS and UE in which the embodimentsof the present invention are implemented.

The BS 800 includes a processor 810, memory 820, and a radio frequency(RF) unit 830. The processor 810 implements the proposed functions,processes, and/or methods. The layers of a wireless interface protocolmay be implemented by the processor 810. The memory 820 is coupled tothe processor 810, and it stores various pieces of information fordriving the processor 810. The RF unit 830 is coupled to the processor810, and it sends a UE-specific SGH parameter to UE.

The UE 900 includes a processor 910, memory 920, and an RF unit 930. TheRF unit 930 is coupled to the processor 910, and it receives aUE-specific SGH parameter. The processor 910 implements the proposedfunctions, processes, and/or methods. The layers of a wireless interfaceprotocol may be implemented by the processor 910. The processor 910 isconfigured to generate a reference signal sequence based on a basesequence for every slot. The base sequence is classified according to asequence-group number and a base sequence number which are determinedfor every slot by a UE-specific SGH parameter indicating whether toperform SGH. The memory 920 is coupled to the processor 910, and itstores various pieces of information for driving the processor 910.

The processors 810, 910 may include application-specific integratedcircuit (ASIC), other chipset, logic circuit and/or data processingdevice. The memories 820, 920 may include read-only memory (ROM), randomaccess memory (RAM), flash memory, memory card, storage medium and/orother storage device. The RF units 830, 930 may include basebandcircuitry to process radio frequency signals. When the embodiments areimplemented in software, the techniques described herein can beimplemented with modules (e.g., procedures, functions, and so on) thatperform the functions described herein. The modules can be stored inmemories 820, 920 and executed by processors 810, 910. The memories 820,920 can be implemented within the processors 810, 910 or external to theprocessors 810, 910 in which case those can be communicatively coupledto the processors 810, 910 via various means as is known in the art.

In view of the exemplary systems described herein, methodologies thatmay be implemented in accordance with the disclosed subject matter havebeen described with reference to several flow diagrams. While forpurposed of simplicity, the methodologies are shown and described as aseries of steps or blocks, it is to be understood and appreciated thatthe claimed subject matter is not limited by the order of the steps orblocks, as some steps may occur in different orders or concurrently withother steps from what is depicted and described herein. Moreover, oneskilled in the art would understand that the steps illustrated in theflow diagram are not exclusive and other steps may be included or one ormore of the steps in the example flow diagram may be deleted withoutaffecting the scope and spirit of the present disclosure.

What has been described above includes examples of the various aspects.It is, of course, not possible to describe every conceivable combinationof components or methodologies for purposes of describing the variousaspects, but one of ordinary skill in the art may recognize that manyfurther combinations and permutations are possible. Accordingly, thesubject specification is intended to embrace all such alternations,modifications and variations that fall within the spirit and scope ofthe appended claims.

What is claimed is:
 1. A method of receiving, by a base station, areference signal in a wireless communication system, the methodcomprising: transmitting a cell-specific sequence group hopping (SGH)parameter to a plurality of user equipments (UEs) in a cell, wherein thecell-specific sequence group hopping parameter is used to enable asequence group hopping for the plurality of UEs in the cell;transmitting a UE-specific sequence group hopping parameter to a certainUE, among the plurality of UEs, wherein the UE-specific sequence grouphopping parameter is used to disable the sequence group hopping, enabledby the cell-specific SGH parameter, for the certain UE; and receiving areference signal, which is generated based on a sequence group number,from the certain UE, wherein the sequence group number is determined bythe UE-specific sequence group hopping parameter.
 2. The method of claim1, wherein the cell-specific sequence group hopping parameter is aGroup-hopping-enabled parameter transmitted through a higher layer. 3.The method of claim 1, wherein the UE-specific sequence group hoppingparameter is a Disable-sequence-group-hopping parameter transmittedthrough a higher layer.
 4. The method of claim 1, wherein the sequencegroup number is identical with each of other sequence group numbers foreach slot.
 5. The method of claim 1, wherein the sequence group numberis defined by a group hopping pattern and a sequence shift pattern ineach of a plurality of slots.
 6. The method of claim 5, wherein thegroup hopping pattern is set to 0 according to the UE-specific sequencegroup hopping parameter.
 7. The method of claim 1, wherein the sequencegroup number is determined by an Equation below:u(f _(gh)(n _(s))+f _(ss))mod 30, where n_(s) denotes a slot number in aframe, f_(ss) denotes a sequence shift pattern configured by a cellidentifier (ID) and a higher layer, and f_(gh)(n_(s)) is a group hoppingpattern which is set to 0 according to the UE-specific sequence grouphopping parameter.
 8. A base station in a wireless communication system,the base station comprising: a radio frequency (RF) unit configured totransmit or receive a radio signal; and a processor coupled to the RFunit, and configured to: transmit a cell-specific sequence group hopping(SGH) parameter to a plurality of user equipments (UEs) in a cell,wherein the cell-specific sequence group hopping parameter is used toenable a sequence group hopping for the plurality of UEs in the cell,transmit a UE-specific sequence group hopping parameter to a certain UE,among the plurality of UEs, wherein the UE-specific sequence grouphopping parameter is used to disable the sequence group hopping, enabledby the cell-specific SGH parameter, for the certain UE, and receive areference signal, which is generated based on a sequence group number,from the certain UE, wherein the sequence group number is determined bythe UE-specific sequence group hopping parameter.
 9. The base station ofclaim 8, wherein the cell-specific sequence group hopping parameter is aGroup-hopping-enabled parameter transmitted through a higher layer. 10.The base station of claim 8, wherein the UE-specific sequence grouphopping parameter is a Disable-sequence-group-hopping parametertransmitted through a higher layer.
 11. The base station of claim 8,wherein the sequence group number is identical with each of othersequence group numbers for each slot.
 12. The base station of claim 8,wherein the sequence group number is defined by a group hopping patternand a sequence shift pattern in each of a plurality of slots.
 13. Thebase station of claim 12, wherein the group hopping pattern is set to 0according to the UE-specific sequence group hopping parameter.
 14. Thebase station of claim 8, wherein the sequence group number is determinedby an Equation below:u=(f _(gh)(n _(s))+f _(ss))mod 30, where n_(s) denotes a slot number ina frame, f_(ss) denotes a sequence shift pattern configured by a cellidentifier (ID) and a higher layer, and f_(gh)(n_(s)) is a group hoppingpattern which is set to 0 according to the UE-specific sequence grouphopping parameter.